A pressure rolling nip is formed between any two mutually parallel cylindrical rolls pressed together. A web passed through the nip with the rolls rotating receives pressure rolling. One example is a paper calender having cooperating soft and hard rolls which pressure roll and, therefore, calender a paper web passed through the nip. A paper calender may include many rolls positioned one on top of another to form a stack.
In the following, only the soft roll and its cooperating hard roll which has a steel surface are involved. The soft roll has traditionally been made by a stack of circular paper disks pressed together under high compression to provide a cylindrical roll with a rolling surface that is ground to a smooth cylindrical surface. The soft surface of the paper roll forms the pressure rolling or calendering nip with its cooperating hard roll having the steel surface, the paper web being passed through this nip. If the web has a defect which is imprinted in the soft roll's surface, it may be necessary to regrind the soft roll's surface. This is expensive and, therefore, undesirable.
An an alternative, the soft roll can be a steel roll covered by a polyurethane layer. The polyurethane is applied to the steel roll's surface in liquid form and is cross-linked on this surface to form a compact coating. This kind of soft roll provides an excellent calendering effect on the paper. If during calendering, a web hard spot or doubled web passes through the calendering nip and forms an impression, the polyurethane layer recovers its original shape after one revolution of the roll so that no trace of the impression remains on the roll's surface. Polyurethane has enormous recoverability after deformation.
On the other hand, the polyurethane layer presents a heating problem. The steel surfaced hard roll and the soft polyurethane covered roll must be pressed together with enough force to cause the polyurethane to deform as it passes through the nip because this in cooperation with the hard roll is what provides the calendering action on the paper web passing through the nip. The polyurethane is not perfectly elastic and as it is deformed by the nip pressure and thereafter resiliently recovers, heat is produced in the polyurethane, the degree of heat depending on the extent of the deformation and recovery which is dependent on the nip line pressure. It is possible to cool the polyurethane layer so that such heating does not build up to temperatures causing degradation or possibly destruction of the polyurethane layer. However, if the nip line pressure, the pressure from end to end of the active portion of the nip through which the paper web is calendered, is not a uniform line pressure throughout, localized heating of the poyurethane layer to degrading or possibly destructive temperatures is possible. Such localized overheating cannot occur if the line pressure is uniform from end to end. Heretofore, this uniformity has been difficult to attain.
There is a kind of controlled deflection roll that permits the nip line pressure to be locally varied at a series of zones extending throughout the nip's length. Such a roll comprises a cylindrical steel shell roll which can form one of the two rolls forming the nip. This shell roll encircles a fixedly positioned beam providing an annular space between itself and the inside of the roll's shell. In this space on the nip side of the shell roll a series of fluid-actuated, radially-acting pressure-exerting units extend axially with respect to the shell, each unit being fixed to the beam and slidingly bearing on the shell's inside so as to exert its individual pressure against a localized zone on the inside of the shell. The shell roll, being made of steel which is elastic, can locally flex so as to deflect at each of the zones towards the other roll to thereby vary the line pressure of the nip formed between it and a cooperating roll, individually at the various zones.
An example of such a controlled deflection roll is shown by the Justus Pat. No. 3,119,324. The fluid pressure actuated units are in the form of cylinders containing pistons having rods extending into sliding engagement with the shell's inside. Each unit has its own individual pressurized fluid supply line. If each unit provides the same piston area for its fluid pressure actuation, and if each unit is supplied with the same fluid pressure, it is academically possible to provide a nip line pressure that is uniform from end to end of the nip. However, under practical operating conditions it is not possible to control the various fluid pressures of the various units so that consistently they are precisely the same, a non-uniform nip line pressure sometimes resulting.
The object of the present invention is to more precisely control the nip line pressure of the nip formed between a hard roll and a soft roll of the polyurethane covered type. It is possible that other plastic coverings may be used for the soft roll and which are equivalents in that they may have essentially the quick recovery and heating characteristics of polyurethane.